Curable silicone compositions which are cured by a hydrosilylation reaction are used to seal, cover, or adhere optical semiconductor elements in optical semiconductor devices. There is a demand for such a curable silicone composition to be able to inhibit the discoloration of silver electrodes or a silver-plated substrate in the optical semiconductor device due to a sulfur-containing gas such as hydrogen sulfide in the air.
For example, a sealing agent for an optical semiconductor element containing from 0.01 to 5 parts by mass of a zinc compound and from 0.01 to 5 parts by mass of a compound having a polyalkylene oxide chain per 100 parts by mass of a curable silicone composition comprising an organopolysiloxane having at least two silicon-bonded alkenyl groups, an organohydrogenpolysiloxane having at least two silicon-bonded hydrogen atoms, and a hydrosilylation-reaction catalyst has been proposed in Japanese Unexamined Patent Application Publication No. 2012-111850A.
However, it was determined that such a curable silicone composition has a problem in that the zinc compound and the silicon-bonded hydrogen atoms in the organopolysiloxane react during storage, which causes the curability to diminish over time. In addition, even such a curable silicone composition has a problem in that it cannot sufficiently inhibit the discoloration of silver electrodes or a silver-plated substrate in the optical semiconductor device due to a sulfur-containing gas in the air.